Effect of exogenous application of salicylic acid on the drought stress responses of Gardenia jasminoides

The alleviative effects of exogenous salicylic acid (SA) on plants against drought stress were assessed in Gardenia jasminoides seedlings treated with different concentrations of SA. Drought stress was simulated to a moderate level by 15% polyethylene glycol (PEG) 6000 treatment. Seedlings exposed to 15% PEG for 14 days exhibited a decrease in aboveground and underground dry mass, seedling height, root length, relative water content, photosynthetic pigment content, net photosynthetic rate (P-n), transpiration rate (T-r), stomatal conductance (G(s)), and water use efficiency. In PEG-stressed plants, the levels of proline, malondialdehyde (MDA), hydrogen peroxide (H2O2), and electrolyte leakage rose significantly, whereas antioxidative activity, including superoxide, peroxidase, and catalase activities, declined in leaves. However, the presence of SA provided an effective method of mitigating PEG-caused physiological stresses on G. jasminoides seedlings, which depended on SA levels. PEG-treated plants exposed to SA at 0.5-1.0 mmol/L significantly eased PEG-induced growth inhibition. Application of SA, especially at concentrations of 0.5-1.0 mmol/L, considerably improved photosynthetic pigments, photosynthesis, antioxidative activity, relative water content, and proline accumulation, and decreased MDA content, H2O2 content, and electrolyte leakage. By contrast, the positive effects were not evident, or even more severe, in PEG+SA4 treatment. Based on these physiological and biochemical data, a suitable concentration of SA, potential growth regulators, could be applied to enhance the drought tolerance of G. jasminoides.